The conversion in chemical reactions is limited by the equilibrium position of the reaction. If the chemical equilibrium of a synthesis reaction is only partially on the side of the products, a single-stage reaction procedure leads only to partial conversion. On the other hand, if the reaction products are continuously discharged from the reactor, continuous conversion of starting materials into products may take place in the reactor.
Many economically important chemical reactions are in practice equilibrium-limited. In chemical synthesis, examples of such reactions are the production of methanol from hydrogen, carbon monoxide, and/or carbon dioxide, or the production of ammonia from hydrogen and nitrogen, known as the Haber-Bosch process. These reactions usually take place in heterogeneously catalyzed fixed-bed or slurry reactors. The starting materials are only partially converted in a single pass through the reactor. The starting material/product mixture is then taken off, the reaction products are usually separated off and unreacted starting materials are recirculated to the reactor inlet.
The recirculation of sometimes large amounts of gas leads to a high outlay in terms of apparatus. The pressure drop in the reactor is compensated for by a recirculation unit. This is usually operated at high temperatures and leads to high costs and a high energy consumption. In addition, inert gases and foreign gases accumulate in the circuit due to the recirculation, which has an adverse effect on the reaction procedure.